Top surface cover upon mess (4)
I believe the answer is:
scum
'top surface cover upon mess' is the definition.
I can't tell whether this definition defines the answer.
'top surface cover upon mess' is the wordplay.
'top' indicates taking the first letters (in a down clue, the first letter is top).
The initial letters of 'surface cover upon mess' is 'SCUM'.
Can you help me to learn more?
(Other definitions for scum that I've seen before include "Dross" , "Impurities on surface of liquid - or bad people" , "Impurities on top of water or worthless people" , "low life" , "Impurities floating on water, or despicable people" .)